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The toll was collected until May 31, 1930.
According to Wikipedia, this 20-para token was valid for "backpacker animals". Because too many examples were minted (10 million) they circulated as money as well.
It's the fare you pay to the tills when you cross Galata Bridge, which is a brigde that conencts two districts, Eminönü to Galata, the Muslim part (Eminönü) to the Non-Muslim part (Galata). Galata is an anciant settlement, originally settled by Italian levantens in early 1100's. It was autonomous until 1700's,and had his own ruler, called "Voivoda of Galata". Today, Galata is an old town district, very touristic indeed.
Yes, the word "Voivoda" is borrowed from Balkans, back then rulers of semi-autonom regions inside Ottoman Empire called "Voivodas". The naming tradition dates back to 1400's, when the Ottomans have both ally voivodas and enemy voivodas in the Balkans.
The old wooden Galata Brigde got burned down in 1992. A new, steel and metal brigde is made to replace it, since it is in a vital, busy traffic section of Istanbul. Cars coming from Eminönü and Aksaray have to cross it to get to Taksim, Beyoglu and Galata. It has a twin brigde, called Unkapani Brigde too, to relieve traffic.